1. Field of the invention
The present invention relates to an image data processing method and apparatus for performing image correction processes for a restored image data file obtained by restoring a compressed image data file.
2. Description of the Related Art
A variety of systems that provide various services for digital image data (hereinafter referred to as “image data”) have emerged in recent years. Examples such as an image data storage and management service system, in which the image data obtained by scanning a negative film, or through a digital camera is stored and managed, and a printing service system, in which image correction processes are performed on image data to obtain an image with desirable image quality before printing, are available today.
In the meantime, most of the systems described above provide services through a network or networks accompanying the rapid popularization of the Internet due to the price reduction of the terminal devices including computers, and the advancement of network technology.
For example, the printing service system described above is configured to print out an image at a printer located, e.g., in a mini-laboratory connected through a network or networks after performing image correction processes on an image data file uploaded to the server from the user terminal, or to store the processed image data in a server with only address information that indicates the location of the file, such as URL, being sent to the lab so that the lab may download the file thereto for printing when such a request is received. In both cases, the delivery of the image data file from the user terminal to the server, or from the server to the printer for printing the image is performed through a network or networks.
In the field of mobile communications, image processing services focusing on personal digital assistants (PDA) are actively being performed along with the popularization and increased features of the PDA, such as cellular phones. Some communication exchange servers, for example, have image correction processing functions, including gradation, white balance, density, and sharpness adjustments, and when relaying an e-mail message between two PDA's, an image data file attached to the message sent from the transmitting PDA is processed by these functions before being sent to the receiving PDA, or the processed file is stored in the server with only address information that indicates the location of the file, such as URL, being sent to the receiving PDA if the PDA cannot receive the attached file, which will be downloaded thereto at a later time when such a request is received. These servers also receive or transmit image data files to PDAs through a network or networks.
These image data files are generally delivered through a network or networks as a compressed image data file in order to reduce the burdens on the terminals and the networks including the reduction in the transmission time. For PDAs, including cellular phones, a particularly tight capacity restriction is imposed and the image data file obtained by photography is stored into the storage section of the device after being highly compressed, since most of these image data files will be sent to other PDAs or computers.
Mainly because of the background and the capacity restriction imposed on the storage medium for storing image data files described above, the image data obtained through a digital camera by photographing a subject, or by reading a film with a scanner or similar devices is usually stored in a storage medium as a compressed image data file.
Thus, most of the image data files inputted to an image data processing apparatus are compressed image data files, and the apparatus will perform the image correction processes after restoring (decompressing) the files. The image correction processes include gradation, white balance, density, and other adjustments. For the gradation adjustment, it is customary that the tonal range or gradation of an image is coarsely or sharply graded to obtain a high contrast image (gradation hardening operation).
Although variety of image data compression technologies are available, including JPEG, GIF, and TIFF, JPEG (including MPEG, a compression technology for animated images based on JPEG, which will apply hereinafter also) is most widely used as the world standard technology for image data compression. In particular, it is being accepted as the common image data compression technology for computers, television, or other media that involve image data processing. JPEG is the image data compression method which is made up of discrete cosine transform, quantization, encoding, and other processing for each image data block. In general terms, however, it is a method based on the frequency characteristic of an image data file, in which a low-frequency component is finely sampled, while the high-frequency component, which is not readily perceivable to human eyes, is more coarsely sampled to obtain a compressed image data file.
The compression technologies described above, including JPEG, are more likely to degrade the image quality as the compression ratio is increased, though in varying degrees depending on the technology used, and the block distortions or artifacts developed in a restored image data file are considered to be an unavoidable price for reducing the size of the image data file. When performing image correction processes, e.g., a gradation adjustment to a compressed image data file, if the adjustment is implemented to a compressed data file having a high compression ratio by setting the image data processing apparatus with a gradation hardening level which is appropriate for an image data file having a low compression ratio and is able to reproduce a comparatively high quality image, the block distortions are further highlighted in the reproduced image, in particular, in the high-frequency component areas, further degrading the image quality.
Similar problems are also found in other image correction processes such as a white balance, density, or other adjustments.